1. Field of the Invention
The present invention relates to a granule for ceramic green body, a ceramic green body and a ceramic sintered body obtained by using the same, and particularly to a granule for ceramic green body which can provide a green body excelling in formability, a ceramic green body and a ceramic sintered body obtained thereof.
2. Description of Related Art
Ceramics have been utilized in various fields inclusive of electronic parts. In this course, ceramics is obtainable by granulating a raw material powder for ceramics together with a binder to provide ceramic granules, forming the ceramic granules to provide a ceramic green body, and sintering the ceramic green is body.
Conventionally, among the various processes which have been utilized for producing a ceramic green body, a dry pressure process has been widely used as a rule. For example, ceramic granules produced either by spray-drying aqueous slurry prepared from a ceramic raw material, a binder, and water by means of a spray dryer or by stirring and mixing a ceramic raw material powder with a binder solution followed by repeating drying and oscillating extruding are formed under pressure to produce a ceramic green body. Sintering the green body thus produced gives a ceramic sintered body.
In order to produce a ceramic green body, the granules for a ceramic green body (hereinafter simply referred to as xe2x80x9cceramic granulesxe2x80x9d) are required to have the following characteristics:
(1) The ceramic granules should have flowability within a suitable range, and a good packing property during the course of packing the ceramic granules into a mold in a uniform manner.
(2) The ceramic granules should be crushed at a low pressure (typically from 29.4 to 147.0 MPa) during the course of molding (hereinafter referred to as xe2x80x9ca crushing property at a low pressurexe2x80x9d).
(3) Ingredients such as fine particles contained in the ceramic granule should not be stuck to a mold or the like (hereinafter referred to as xe2x80x9canti-sticking propertiesxe2x80x9d).
(4) The ceramic granules are not collapsed during the course of the storage, transportation, or agitation when they are packed into a mold, or due to collision with each other (hereinafter referred to as xe2x80x9can anti-collapse propertyxe2x80x9d).
(5) The ceramic granules have suitable bulk density so that they flow out of the mold during the course of packing them into a mold, or of molding (hereinafter referred to as xe2x80x9ca mold-packing propertyxe2x80x9d)
Particularly, the ceramic particles are required to possess conflicting characteristics, i.e., xe2x80x9cthe crushing property at a low pressurexe2x80x9d and xe2x80x9cthe anti-collapse propertyxe2x80x9d.
Various processes have been suggested in order to satisfy these requirements. For example, Japanese unexamined Patent Publication No. 5-159918 and Japanese examined Patent Publication No. 7-17460 disclose processes for improving the flowability and the crushing property at a low pressure utilizing specific dispersants in the preparation of an aqueous slurry.
Japanese examined Patent Publication No. 3-31660 and Japanese unexamined Patent Publication No. 10-59776 disclose processes for improving the flowability and the crushing property at a low pressure of the ceramic granules by decreasing the segregation of the binder.
According to these processes, it is disclosed that ceramic granules having improved flowability, relatively a good mold-packing property, and a crushing property at a low pressure of the ceramic granules can be obtained.
However, these techniques have the following drawbacks:
(1) Since these techniques concern the improvement of aqueous slurry, the granulation process is restricted to the spray-drying process using a spray dryer, leading to poor popularization.
(2) Although the flowability and the crushing property at a low pressure of the ceramic granules are somewhat improved, the something to further improved has been still left. Also, the green body produced by the use of such ceramic granules has insufficient dimensional accuracy. Further, these ceramic granules are not suitable for producing a ceramic green body having a complicated shape.
(3) The ceramic granules are sometimes collapsed during the course of the storage, transportation, or packing them into a mold.
(4) When being molded in a mold, the fine ceramic particles are stuck to the mold, which in some cases makes it impossible to continuously produce ceramic green bodies.
Consequently, a first object of the present invention is to provide a ceramic granule excelling in the flowability, the mold-packing property, and the anti-sticking property, and having a well-balanced properties of conflicting properties of the crushing property at a low pressure and the anti-collapse property, and capable of continuously producing ceramic green bodies.
A second object of the present invention is to provide a highly dense ceramic green body having a high molding density and a sintered body obtainable by sintering the green body.
We have made an assiduous study in research in light of the above situation. As a result, it has been found that the use of a polyvinyl alcohol having specific characteristics attains these and other objects, leading to the present invention.
According to the first aspect of the present invention, there is provided a ceramic granule for a ceramic green body containing a raw material ceramic powder and a binder component comprising a polyvinyl alcohol, said polyvinyl alcohol having an average saponification degree of not less than 90.0 mol % and not more than 98.0 mol %.
In this ceramic granule, the polyvinyl alcohol component preferably comprises a polyvinyl alcohol having an average saponification degree of not less than 94.5 mol % and not more than 15 97.5 mol %. Also, the amount of the polyvinyl alcohol added is preferably not less than 0.2 parts by weight and not greater than 10 parts by weight based on 100 parts by weight of the ceramic raw material powder.
According to the second aspect of the present invention, there provide a ceramic green body obtainable from such a ceramic granule for a ceramic green body, and according to the third aspect of the present invention, there provided a ceramic sintered body obtainable by sintering such a ceramic green body.